Surface relief structures, commonly referred to as surface gratings, are currently important and of interest in several technological areas. For example, such structures are important in integrated optics, ultraviolet spectroscopy, surface wave devices, negative resistance devices and distributed feedback lasers. The surface structure may have a constant or variable spatial period and while most contemplated structures have straight lines, curved line structures are useful for some purposes. The spatial period used in a device depends upon both the device type and desired characteristics. However, the spatial period in typical devices is generally less than several microns and, for many purposes, submicron periods are desirable.
Several methods have been used to fabricate surface relief structures and will be briefly described. The first method is described in Applied Physics Letters, 23, pp. 154-155, Aug. 1, 1973, and uses two interfering beams from a He-Cd laser emitting at 3250 Angstroms to create a periodic surface structure with a period limited to .DELTA./2n where .DELTA. is the exposure wavelength and n is the refractive index of the recording medium. Spatial periods smaller than .DELTA./2 may be produced by using a prism and index matching fluid which reduces the wavelength in the exposure medium. A prism and an indexing matching fluid having a refractive index of approximately 1.5 produced periodic surface structures having a period of approximately 1100 Angstroms. However, smaller spatial periods using this method will most likely require coherent light sources having wavelengths shorter than that of the He-Cd laser used and such light sources are not readily available. This method produces a grating with the desired spatial period directly. Numerous profiles may be easily fabricated with this method and the ion beam erosion technique described in Applied Physics Letters, 35, pp. 500-503, Oct. 1, 1979. If smaller spatial periods are needed a first or parent structure having a first spatial period may be used to produce a second or child structure having a second and smaller spatial period. The second method, illustrating this approach and referred to by the authors as spatial period division, is described in Journal of Vacuum Science Technology, 16, pp. 1949-1952, (Nov./Dec., 1979). This method relies on the existence of intensity maxima in the near field diffraction pattern of a grating mask to produce periods smaller than that of the mask. X-ray radiation was used to produce a child grating having a period of 984 Angstroms from a parent grating having a period of 1968 Angstroms. The method, however, involves several processing steps and depends rather critically on the spacing between the parent and child gratings. Many of the contemplated devices using surface gratings will require spatial periods less than 1000 Angstroms and simpler methods of making such gratings are desirable.